librpc/ndr: consolidate string conversion logic in ndr_pull_string

librpc/ndr: consolidate string conversion logic in ndr_pull_string

Reduce the amount of duplicate code in ndr_pull_string by moving the
almost duplicate conversion calls and their corresponding NDR pulls and
checks to a single location.  In the place of the removed calls is logic
allowing the conversion to be generalized, and and any specific
pulls/checks that do not apply to the general case.

This is similar to what has already been done in the switch statement
for ndr_push_string.

Signed-off-by: Sean Finney <seanius@seanius.net>